1. Field of the Invention
The present invention relates to a bumper beam for reinforcing bumpers for automobiles.
2. Description of the Related Art
In general, an automotive bumper is composed of a bumper beam that is coupled to a body of an automobile and maintains strength of the bumper and a resin-made surface skin attached to the bumper beam to improve external appearance. Efforts have been made to lighten bumper beams to reduce fuel consumption, and in recent years bumper beams are in many cases formed from light alloys. For example, a bumper beam 60 shown in cross section in FIG. 12 is one example of a bumper beam extruded from aluminum alloy and has a hollow structure of “two-adjacent-square” cross section. In other words, the bumper beam 60 is comprised of top and bottom walls 61 and 62 parallel to each other, parallel lateral walls 63 and 64 that are perpendicular to the top and bottom walls 61 and 62, and a connection rib 65 provided intermediately so as to divide the lateral walls 63 and 64 into two.
In practical use, the bumper beam 60 is mounted through a side member 66 on the front or rear of an automobile body 67, and the lateral wall 63 constitutes, in a collision, a collision side lateral wall that receives an impact force F from a leftward direction as indicated by an arrow in the figure. Thus, of the members constituting the structure of the “two-adjacent-square” cross section, the lateral wall 63 is made the thickest. In the example of FIG. 12, the top wall 61, bottom wall 62 and connection rib 65 are formed with the same thickness to provide a structure whereby to equally receive and lessen the impact force from the leftward direction in the figure.
With a view to lightening, such a bumper beam is made of a high tensile aluminum alloy or the like. The bumper beam is usually attached with a cushioning made of foamed material or the like and its surface is covered with a bumper cover.
A bumper beam, when an external impact force is applied in an automobile collision or the like, absorbs the impact energy through plastic deformation of its material, thereby to avoid damage to other members and at the same time secure safety of an automobile occupant, and thus is an essential member.
Note that as patterns of automobile collisions, there can be mentioned a pattern in which a wall-like obstacle collides at a relatively high velocity against an overall wall surface of a bumper beam, and a pattern in which a columnar obstacle collides at a relatively low velocity against a part of a wall surface of a bumper beam. In many collisions of the former pattern, the collision energy involved is so great as to cause injuries to an automobile occupant as well as buckling damage to the bumper-beam mounting member. To cope with this, a bumper beam is desired which is capable of undergoing gradual deformation and collapse to absorb a large amount of collision energy. On the other hand, in many collisions of the latter pattern, the collision energy involved is seldom so great as to cause injuries to an automobile occupant and damage to the bumper-beam mounting member. In this case, such a bumper beam is desired which has high rigidity to resist deformation due to the load of collision rather than absorbs collision energy through deformation and collapse.
A bumper beam is required to have an increased bending rigidity of its sections and energy absorbing ability in case of bending, while at the same time to have lighter weight. A proposal has been made, for example, in Japanese Patent Application Unexamined Publication No. 8-80789 (see page 1; FIG. 2) that improves these characteristics through an improvement of the cross sectional shape of a bumper beam.
Here, a bumper beam is disclosed which is made of an aluminum alloy section of rectangular cross sectional shape uniform in its lengthwise direction and is mounted, at both ends of its automobile-body-facing wall surface, on an automobile body so as to have a vertical wall surface relative to the direction of collision. In this bumper beam, both corners of the aluminum alloy section located on the automobile body side are curved with a radius of curvature R which is 2.5 or more of the wall thickness.
More specifically, as shown in FIG. 13, the proposed bumper beam 70 is made of an aluminum alloy section sheathed in a bumper cover and has an automobile-body side wall surface 71a supported through a side member 74 on an automobile body 72. The aluminum alloy section as mentioned above has a rectangular “two-adjacent-square” cross-sectional shape which is uniform in the lengthwise direction and is composed of a pair of horizontal ribs 71b and 71b, vertical ribs 71a and 71a connected to both ends of the horizontal ribs 71b and 71b, and a reinforcement rib 71c interconnecting the vertical ribs 71a and 71a. 
In the bumper beam 70, it is arranged that the vertical ribs 71a and 71a are perpendicular to the direction of collision and the horizontal ribs 71b and 71b are parallel to the direction of collision. The corners 71d and 71d on the side of the automobile body 72 are curved with a radius of curvature R which is 2.5 or more of the wall thickness within the confines of ⅙ or less of the length of the vertical ribs 71a and 71a. The corners 71e and 71e of the bumper beam 70 on the collision side are curved with a radius of curvature r approximately as large as the wall thickness. With the thus constructed bumper beam 70, at the time of collision against a barrier, the curved corners 71d and 71d are positioned at a starting point of buckling, thereby to accelerate buckling and effectively absorb collision energy while suppressing the load generated. Furthermore, at the time of collision against a pole, the curved corners 71d and 71d are positioned on a side opposite a starting point of buckling, thereby to allow a large load to be generated. The reason for limiting the radius of curvature R to ⅙ or less of the length of the vertical ribs 71a and 71a is that, if the radius of curvature R exceeds ⅙, it becomes difficult to mount on the side member 74 and a reduction is made in the energy absorbed.
It is hoped that such a structure realizes both characteristics as needed to cope with the above-mentioned two patterns of collisions, i.e., the characteristic of undergoing gradual deformation and collapse to absorb a large amount of collision energy and the characteristic of having rich rigidity to resist deformation due to the load of collision.
If a bumper beam, however, is too strong, damage will be caused to a side member, the mounting hardware for the bumper beam, along with the buckling of the bumper beam. The side member will be damaged by the maximum load generated at the moment of collision.
For example, with the bumper beam as shown in cross section in FIG. 12 which has all the corners bent at right angles, the average load during the collision-caused plastic deformation of the bumper beam from 3.5-4.5 mm is approximately 50 kN as shown in FIG. 14, while a maximum load of 250 kN is generated during the plastic deformation of the bumper beam of approximately 0.5 mm, before the amount of displacement of the bumper beam reaches 1 mm immediately after the collision. The deformation proceeds under a substantially constant crushing load after deformation up to approximately 2 mm. In this case, the maximum load is 5.88 of the average load.
If this maximum load can be reduced without impairment of the energy-absorbing ability of the bumper beam, collision energy will be absorbed, without damaging the side member, only by deformation and collapse of the bumper beam.
Conventionally, the matter of concern was the relationship between the maximum load and the energy absorbed during the plastic deformation of the bumper beam from 3.5 to 4.5 mm, i.e., at the time when the generated load shows no substantial fluctuations, and no attempts have been made to reduce the maximum load generated at the moment of collision.